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Apr 7, 2014

How To Build A Quantum Telescope

Quantum optics has revolutionised microscopy. Now astronomers are planning to jump on the quantum bandwagon

The diffraction limit is astronomy’s greatest enemy. The resolution of all telescopes is limited by factors such as imperfections in the optics and turbulence in the atmosphere. But these can be overcome using better equipment, adaptive optics and by getting above the atmosphere.

But there is one limit that astronomers cannot overcome because it is set by the laws of physics—the diffraction limit. Every telescope on the planet, and all those orbiting it, are limited in this way. And until recently there was no known way to beat it.

Now physicists have begun to develop various quantum techniques that can overcome the diffraction limit, at least in the lab. These techniques have begun to revolutionise microscopy, where the light source can be carefully controlled. But they have yet to be considered for astronomy because astronomers have little, if any, control over the sources of the light they are interested in.

Today, however, Aglae Kellerer at the University of Durham explains how to build a quantum telescope. She says quantum techniques could dramatically improve the resolution of telescopes by beating the diffraction limit for the first time.

When light from a point source enters a lens, it bends. This bending causes the light to spread out so that it interacts with itself generating an interference pattern.

The result is that a lens always resolves a point source as a bright disc surrounded by concentric circles of light, a pattern known as an Airy disc. The size of this disc, which is determined by the wavelength of light and the size of the lens, determines the ultimate resolution of the instrument.

In practice, most telescopes are limited by other factors, in particular, turbulence in the several kilometres of atmosphere above them. But techniques such as adaptive optics, which iron out the effects of turbulence, are allowing telescopes to get much closer to the diffraction limit.

So how to go further? Kellerer’s idea is to exploit the strange effects of quantum mechanics to improve things. Entanglement, for example.

Entangled photons are so deeply linked that they share the same existence. Measure one and you automatically influence the other. That gives you information about the other photon, regardless of its distance from you.

Last month, physicists used this idea to build the world’s first entanglement-enhanced microscope that dramatically increases its resolution over purely classical instruments. They created entangled photons and used one to illuminate the object. The second photon can then give them information about the first that they use to increase the resolution of the resulting image.

There’s an obvious problem in employing these kinds of techniques in astronomy—the photons of interest aren’t under your control, having travelled many lightyears from their astrophysical source.

But Kellerer says there is a way round this. Her idea is to use the astrophysical photons to stimulate the production of an entangled pair, inside a telescope. The first of this pair then hits the detector, generating an image. But the other can be used to increase the information known about the first, thereby increasing the resolution and beating the diffraction limit.

That’s an interesting idea that has the potential to significantly increase the resolution of conventional telescopes. In fact, Kellerer has simulated the effect of this process on computer to enhance the resolution of a conventional astronomical image by a factor of six.

But building an instrument that works in this way will be hard and the devil is in the detail.

The first problem is in producing entangled photons in the first place. Kellerer’s idea is to use a crystal of excited atoms that emit entangled photons when stimulated by passing astrophysical ones.

The problem is the efficiency of such a process. With so few astrophysical photons to play with, any that are lost due to inefficiencies are a serious problem.

Then there is the problem of spontaneous emission. Excited atoms have a nasty habit of emitting photons, even when they are not stimulated by passing photons. That’s noise, which could end up overwhelming the signal from the photons astronomers are interested in.

Both of these problems can be minimised but they cannot be removed completely. The question is whether the advantages of this technique can be made to outweigh the disadvantages in a real device.

There’s only one way to find out, of course. Today, the technology required to test this idea is in its infancy. But there’s good reason to think that significant advances will be made in the near future.

And that means that future telescopes could be very different from the ones we have today. It’s a sobering thought that if the pioneers of telescopic imaging were around today, they would find that the world’s best telescopes work in more or less exactly the same way as their own from 400 years ago.

But Kellerer’s approach would be entirely alien to them and could finally take astronomy into a new era of quantum imaging.